Noncontact position measurement systems using optical sensors

ABSTRACT

Optical and computational components are combined to form high precision, six degree-of-freedom, single-sided, noncontact position measurement systems. Reflective optical targets are provided on a target object whose position is to be sensed. Light beams are directed toward the optical targets, producing reflected beams. Electrical signals are produced by movements of reflected beams across position-sensitive detectors, such as lateral-effect photodiodes. The signals are transformed to provide measurements of translation along and rotation around three nonparallel axes which define the space in which the target object moves.

The Government has certain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The claimed invention relates to methods and apparatus to measureposition changes, i.e., displacements and rotations, of a sensed objectin space, in six degrees of freedom (six-DOF).

2. Optical Sensor Applications

The assembly of mechanical items often requires that relatively largeparts be fit together with very small dimensional tolerances. Such taskspose a challenge for automation because the scales involved in theprocess typically span several orders of magnitude. Generally, thelarger and more massive the parts to be assembled, the more substantialthe assembly machine required to perform the task. As the assemblymachine increases in size, however, the accuracy of open-loop controlmethods generally decreases.

One can significantly improve the accuracy of the assembly process byusing closed-loop control techniques, but only if real-time sensoryinformation on the relative location of parts to be assembled isavailable. Additionally, the accuracy of the closed-loop assemblyprocess is limited by the accuracy of the sensed information. Hence,operations requiring high accuracy placement of parts must use highresolution sensors in closed-loop control systems. In some cases, thesensors must be multidimensional, detecting both translation androtation of parts.

LATERAL EFFECT PHOTODIODE SENSORS IN PRIOR SYSTEMS

Lateral effect photodiodes have been used in the past in systems formonitoring position changes of an object, but known systems discussed inthe literature are either limited to two-dimensional information, or arenot realizable in a single plane. For example, an earlier-described,four-dimensional system for inspection of tappet bore positions in adiesel engine used two lateral effect photodiodes placed on orthogonalsurfaces. It effectively comprised two independent two-dimensionalposition sensing sub-systems. Such a system, however, would not answerthe need for precise position information in six-DOF.

MULTI-DIMENSIONAL POSITION SENSING

If, for example, it is desired to place a toothed gear with a spindleinto a housing, one might need to sense and control part motionparameters in six-DOF, i.e., three rotational and three translationalcoordinates. The multi-dimensional position sensing problem thusencountered is very complex for two major reasons.

First, in assembly operations it is frequently unacceptable for thesensor to occupy more than one side of a part (1) because it could limitthe ability of the assembly machine to position the part, and (2)because different part surfaces may not be orthogonal or even flat.

Second, cross talk between sensing channels tends to occur during use ofmultiple small sensor systems in close proximity.

The above limitations impose substantial penalties on sensor systems inthe form of slow performance, high cost and unnecessary complexity. Inview of the resulting disadvantages, prior art systems have failed tomeet one or more of the following sensor system design criteria.

HIGH-PRECISION SENSING SYSTEM REQUIREMENTS

Six requirements must generally be met in high precision sensing forpart assembly:

(1) The sensors must have high resolution. Typical assembly tolerancesare of the order of 1-100 μm, so the sensor should be capable oftranslational resolutions of less than 1 μm and angular resolutions inthe μrad range.

(2) Sensing must be noncontact so that delicate parts are neitherdamaged nor otherwise affected by the presence of the sensor.

(3) The sensing system must be light-weight, compact, and versatile in awide range of assembly and machining operations.

(4) The sensing system must provide real-time continuous displacementand rotation (position) information usable as part of a feedback controlscheme. Long delays associated with data processing are unacceptable,because they lower productivity and may result in system instability.

(5) The sensing system must be capable of simultaneous,multiple-coordinate sensing.

(6) The sensing system should not be prohibitively expensive.

TRADITIONAL APPROACHES TO POSITION SENSING

Four main sensing approaches have generally been considered fornoncontact displacement sensing: ultrasonic echo-ranging, inductancesensing, capacitance sensing, and optical sensing. Ultrasonic sensingcan be ruled out for high-precision work on the basis of the accuracyneeded for precision assembly tasks. For example, to obtain a 1 μmwavelength, one would require a 343 MHz sound wave. Regrettably, soundat such high frequencies is rapidly attenuated in air.

Inductance and capacitance approaches to displacement measurementsensors are also difficult to apply to high-precision work because (1)they are limited to particular classes of materials, (2) they are highlynonlinear, and (3) they are inherently one-dimensional in nature.

In contrast, optical methods in the form of vision systems offeraccurate noncontact position sensing. Though frequently more expensive,larger, and computationally slower than desired, vision systems havenonetheless been adopted in many applications as the best availablealternative.

SUMMARY OF THE INVENTION

The present invention improves on previous vision systems for measuringposition (thus permitting determination of displacement and rotation) ofa sensed object by incorporating their most desirable features (accuracyand noncontact operation), while adding new features not previouslyavailable together in six-DOF vision systems (substantially real-timeoperation and relative simplicity). By applying relatively inexpensiveoptical elements in a uniquely compact configuration, sensor systems ofthe present invention operate in a noncontact mode, in substantiallyreal time, and from one side of a sensed or target object, deliveringdisplacement measurement information on a sensed object (target) in upto six-DOF. If desired, all sources and sensors in systems utilizing thepresent invention may be located in a single plane, although otherconfigurations are also within the scope of this invention.

Whatever the embodiment and configuration chosen, sensor systems of thepresent invention employ light beams reflected from plane surfacesaffixed to or prepared as integral parts of the object being sensed. Byknowing the position of each projected and reflected beam, and therelative locations of the optical sensors and emitters, the set of beammovements with respect to a sensor may be transformed by calculationinto changes of position (displacement and/or rotation) of the sensed(target) object in a space which may be defined by three nonparallelaxes. In preferred embodiments, the three axes are orthogonal, andcalculations to determine position (displacement and rotation) areperformed by a digital computer.

The invention is distinguished from prior sensing systems for threereasons. First, in preferred embodiments it comprises a six-DOF sensorsystem, rather than the one- or two-DOF systems discussed in theliterature and available commercially. Second, unlike a vision-typesystem, it may be based on compact and relatively inexpensive sensingelements. Third, the invention may use analog sensing elements ratherthan digital. This has the advantage of allowing for potentialimprovements in resolution while reducing the number of systemparameters which must be manipulated. The novel system designdramatically reduces both the volume of data generated and the amount ofcomputation required to produce the desired information. In summary,preferred embodiments of the claimed invention uses novel combinationsand configurations of standard optical components to achieve moreaccurate, faster, and more economical operation than standard visionsystems for position sensing.

Among the more important optical components of preferred embodiments ofthe present invention are silicon lateral-effect photodetectors. Theymay be used in an inherently normalizing strategy so that results areindependent of light intensity (making it unnecessary to calibrate foreach reflecting surface). In particular, lateral-effect photodiodes maycomprise each position-sensitive detector (PSD) in preferred embodimentsbecause the photodiodes demonstrate high resolution and accuracy andbecause they are available in a form which yields four signals perdevice. The signals may be combined to develop two-dimensionalinformation, making the devices appropriate for multiple-coordinatesensing. In preferred embodiments, PSD's are positioned to interceptlight beams reflected from reflectors or reflecting surfaces on thesensed object, specific positions being determined by the signals neededto calculate position of the sensed object in each embodiment of thepresent invention.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the herein described advantages and featuresof the present invention, as well as others which will become apparent,are attained can be understood in detail, more particular description ofthe invention summarized above may be had by reference to the embodimentthereof which is illustrated in the appended drawings, which drawingsform a part of this specification.

It is to be noted, however, that the appended drawings illustrate onlyexemplary embodiments of the invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 illustrates a six-DOF position measurement system comprising twosubstantially parallel beams which are used to measure out-of-plane(OOP) position changes, plus a single beam to measure in-plane positionchanges.

FIG. 2 illustrates a six-DOF position sensor system comprising paralleland nonparallel beams to measure OOP position changes, plus a singlebeam to measure in-plane position changes.

FIG. 3 illustrates a six-DOF motion sensor system comprising twononparallel beams to measure OOP position changes, plus a single beam tomeasure in-plane position changes.

FIG. 4 illustrates a six-DOF motion sensor comprising a single beam tomeasure both OOP and in-plane position changes.

FIG. 5 illustrates a six-DOF position measurement system substantiallyas in FIG. 1 except that OOP movements are sensed on a single PSD andthe light source for the in-plane measurement section is not in theplane of the PSD's.

FIG. 6 is a block diagram illustrating the process of multiplexing twolight beam positions on the same PSD, followed by demultiplexing signalsfrom the PSD to recover the original light beam positions.

FIG. 7 is a block diagram illustrating an embodiment including acomputer that generates an output containing information regardingposition or motion of a target object.

DEFINITIONS

DOF degree-of-freedom

I_(i) the photocurrent to electrode i (i varying from 1 to 4)

I_(ij) the photocurrent of the ith electrode of the PSD generated as aresult of light source j

LED light-emitting diode

L_(i) the length of each electrode

m the number of beams irradiating the PSD

n the normal to the boundary of the PSD electrode

OOP out-of-plane

PAM pulse amplitude modulation

Φ out-of-plane rotation around y-axis

φ_(r) the potential distribution caused by the reverse biasing voltage

φ_(s) the potential distribution caused by the photovoltage

φ_(sj) the potential distribution induced by source j

planar in-plane

PSD position-sensitive detector

Ψ out-of-plane rotation around x-axis

Θ in-plane rotation around z-axis

V_(r) reverse-biased voltage

x-axis one of two axes for planar translation

y-axis one of two axes for planar translation

z-axis axis for out-of-plane translation

DETAILED DESCRIPTION

Five Preferred Embodiments

In the four embodiments of FIGS. 1, 2, 3 and 5 that will be describedherein, the six-DOF position sensing system of the instant inventioncomprises two sections: 10 and 70, 11 and 70, 12 and 70, or 13 and 68respectively, each having its own assembly of light sources, reflectorsand sensors. In a fifth embodiment (FIG. 4) the functions of the twosections are combined in one section.

Considered separately, the embodiments of the first section, labeled 10,11, 12 and 13 in FIGS. 1, 2, 3 and 5 respectively, are for measurementof OOP motion. The second section 70 is for measurement of in-planemotion and is identical in FIGS. 1, 2 and 3. Section 68 (FIG. 5) is alsofor measurement of in-plane motion and differs from section 70 in FIGS.1, 2 and 3 in that in section 68, light beam 77a, projected by emitter75a, is deflected by mirror 78 to form a deflected beam 79 beforereflection from substantially point reflectors 73, 74. Although elementsof the invention will in most instances be configured so that lightsources and sensors are coplanar (FIGS. 1, 3 and 4), FIGS. 2 and 5illustrate as examples embodiments wherein all sources and sensors arenot coplanar.

In each of the embodiments represented in FIGS. 1, 2 and 3, the OOPsection comprises two light sources or emitters (called first and secondbeam emitters and numbered 21 and 22, 35 and 36, and 55 and 56respectively), while the in-plane section comprises a single (targetbeam) emitter 75. First and second emitters emit first and secondprojected beams (23 and 24 in FIG. 1, 37 and 38 in FIG. 2, and 60 and 61in FIG. 3), while the target beam emitter emits a target beam 77. Insection 70, there is a single (target beam) emitter 75 projecting a beam77 which on reflection from 73, 74 (substantially point reflectors)becomes reflected-target beams 80 and 81 in FIGS. 1, 2, and 3. In FIG.5, emitter 75a projects target beam 77a, which is deflected by mirror 78to form deflected beam 79 prior to reflection from 73, 74 asreflected-target beams 80 and 81. In the embodiment of FIG. 4, a singleemitter 75 projects beam 77 to substantially point reflectors 71, 73 and74, and reflected-target beams are identified as 80, 81 and 84 in FIG.4. Further as to terminology in the present application, light beamsprojected from first and second emitters are, after reflection from thetarget object, first and second reflected beams respectively. Note,however, that a projected beam (e.g., a first projected beam) may besplit into a plurality of beams. In that case, each of the plurality ofbeams will, on reflection from the target object, comprise a reflectedbeam (e.g., a first reflected beam), and each individual beam of theplurality reflected will be a split reflected beam (e.g., a split firstbeam). A single target beam may also produce a plurality of reflectedbeams (as in FIGS. 1-3 and 5) or perhaps three reflected beams (as inFIG. 4) which will be referred to as target-reflected beams. If thetarget beam 77 itself is deflected prior to reflection as in FIG. 5(where the target beam is 77a), individual reflected beams 80 and 81will be deflected target-reflected beams. Further, a target beam ispreferably collimated, and a deflected beam impinging on the pointreflectors will then be a deflected-collimated target beam.

The plane which serves as a reference for motion measurement is thereference plane 72, which in preferred embodiments may be substantiallyparallel to the plane in which reflectors 73 and 74 are positioned. Thereference plane 72 is drawn coplanar with OOP reflectors 73 and 74 inFIGS. 1 and 5, 43 and 44 in FIG. 2, and 59 in FIG. 3 for illustrativepurposes only, i.e., for illustrative purposes only, the OOP reflectorsin FIGS. 1-3 and 5 may be considered to be portions of a reflectivesurface 69 of plane 72. Plane 72 is also drawn to contain the x and yaxes of the three-axis set (x, y and z) which defines the space in whichthe sensed object moves. In preferred embodiments the axes areorthogonal, although this condition is not essential for operation ofthe invention.

Referring to FIG. 7, in preferred embodiments each emitter 130 anddetector 132 is connected to a digital computer 134 which controls theoperation of the emitter and detector, and which calculates the positionand/or the motion of the target object based on information receivedfrom the detector 132 which detects the position of a light beamreflected from reflector 136, which is disposed on the surface of thetarget object. The calculations performed by computer 134 may comprisethe transformations set forth elsewhere in this specification.

Measurement of OOP Parameters z, φ, and Ψ

The OOP parameters measured by the first section 10, 11, 12, 13 aretranslation along the z axis, and rotation about the x and y axes, whichare Ψ and Φ respectively. The in-plane parameters measured by secondsection 70, 68 are translation along the x and y axes, and rotationaround the z-axis (θ). Although the second section 70, 68 uses separatesensors and sources in the four embodiments illustrated in FIGS. 1, 2, 3and 5, the final determination of these parameters is dependent on theearlier-determined OOP parameters in each case. This means that errorsin any OOP parameter determination may produce errors in the in-planeparameter determinations, whereas OOP motion determinations areinsensitive to in-plane motion (assuming that the reflectors extendbeyond the sensing range in the x and y directions).

Four separate embodiments of the OOP sensing section, labeled 10, 11, 12and 13, are illustrated on the left sides of FIGS. 1, 2, 3 and 5respectively. Distinctly different emitter/sensor arrangements are usedin each illustrated preferred embodiment; in the embodiments of FIGS. 1and 5, the two light beams 23, 24 projected from emitters 21, 22 aresubstantially parallel, while in the embodiments of FIGS. 2 and 3,emitted beams 37, 38, 60, 61 are nonparallel. In all four cases, eachPSD 29, 30, 45, 46, 47, 57, 58, 18 produces four signals related to thetwo-dimensional position of each light beam striking it. Thus, there isadequate information to calculate the three OOP parameters. As long asthe target surface 69 is reasonably reflecting, none of thesearrangements requires modification of the object to be sensed. It shouldbe noted that the reflective surfaces used in OOP sensing sections maycomprise a continuous and planar surface (a first reflective surface) ora plurality of reflective surfaces. In the latter case, individualsurfaces are referred to collectively as a first reflective surface, andmay be, but need not be, parallel or coplanar.

Referring to FIGS. 1 and 5, the OOP sensing section 10 shows two of thepreferred sensor and target geometries for measurement of the OOPparameters z, Φ, and Ψ; in these Figures and throughout thisapplication, it is assumed that the targets (reflectors) may be treatedas flat light-reflecting planes or surfaces. In the OOP portion 10 ofthe FIG. 1 and FIG. 5 sensor, the two light sources or emitters 21, 22are not in the reference plane 72 but project substantially parallellight beams 23 and 24 which reflect obliquely off portions 25, 26 ofreflective surface 69 to impinge on two PSD's 29, 30 (in the embodimentof FIG. 1) or one PSD 18 (in the embodiment of FIG. 5). In the latterembodiment, positions of the two light beams on PSD 18 are separatelydetected through multiplexing/demultiplexing techniques discussed below.

PSD's 29, 30, 18 are not in the reference plane but are on the same sideof the reference plane as emitters 21 and 22. Note that portions 25, 26of reflecting surface 69 are illustrated as substantially parallel to orcoplanar with the reference plane 72, but need not be. Further, surfaceportions 25, 26 themselves are illustrated as substantially parallel orcoplanar but need not be as long as the entire cross-section of eachreflected beam 27, 28 is incident on the PSD's 29, 30, 18 of therespective embodiments and the relation of surfaces 25, 26 to each otheris known.

The choice of emitters 21, 22 is a critical factor in determining theworking range of the sensing system. The combined effects of spreadingof the light beam and limitation of the PSD effective area will set themaximum working range of the sensor system for both OOP and in-planeparameters. The preferred embodiment for this invention is any LED whoseintensity may be adjusted and whose light beam is confined to a narrowangular range (up to ±15 degrees).

Through careful placement of PSD's 29, 30, 18 and processing of the PSDsignals, the influence of the z measurement on the difference signal canbe removed. An advantage of this approach is that it results inrelatively simple transformations to obtain Φ and Ψ from the normalizedoutput signals of each PSD. The z coordinate of target position may bedetermined from the OOP signals of any one of the PSD's 29, 30, 18. Thenthe values of Φ and Ψ may be determined. Hence the disadvantage of thisapproach is that the accuracy of the angular-coordinates determinationis dependent upon the accuracy with which z is determined.

An alternative to the substantially parallel light beam geometryillustrated in FIG. 1 is to introduce skewing between the projectedlight beams (see FIGS. 2 and 3). In these embodiments, a pure z axistranslation of the target will cause relative motion between the lightspots impinging on PSD's (45, 46, 47 in FIG. 2 and 57, 58 in FIG. 3). Asa result, the inverse transformations to determine parameters z, Φ and Ψare more complex, but all determinations are made directly from the PSDsignals. Further, errors in alignment of the emitters and PSD's can becompensated, whereas in the substantially parallel beam case (FIG. 1),precise alignment of emitters and PSD's is critical.

Referring to OOP section 11 of FIG. 2, emitter 35 projects a light beam38 to half-silvered plane mirrors 39 and 40, each of which deflect andsplit incident beam 38, transmitting a portion of the beam as indicatedand deflecting a portion of the beam. In preferred embodiments, aplurality of such mirrors, disposed so that mirror planes are parallel,are employed to produce a plurality of parallel split beams whichimpinge on a reflective surface of the sensed object. In FIG. 2, aportion of incident beam 38 is deflected from mirror 39 toward portion43 of reflective surface 69, the path being indicated by beam 41.Another portion of beam 38 is transmitted through mirror 39 to mirror40, from which a portion of it is deflected toward portion 44 ofreflective surface 69 along a path indicated by beam 42. Light incidenton portions 43, 44 of reflective surface 69 travels in substantiallyparallel paths 41, 42 and is reflected back to half-silvered mirrors 39,40, where a portion of each beam is transmitted to PSD's 45, 47 alongpaths indicated by beams 48 and 49. The planes of portions 43, 44 ofreflective surface 69 are preferably coplanar, but need not be coplanaror even parallel, as long as their relative locations are known.

Another portion of the OOP section 11 of FIG. 2 includes emitter 36,from which a beam of light 37 is projected obliquely to portion 43 ofreflective surface 69 within or substantially parallel to referenceplane 72, portion 43 being shown within plane 72 in FIG. 2 forillustrative purposes only. Reflected beam 50 is incident on PSD 46.

FIG. 3 illustrates another alternative to the substantially parallellight beam geometry illustrated in FIG. 1. In this embodiment, emitters55 and 56 project nonparallel beams of light 60 and 61 obliquely towardreflective surface 59, while reflected beams 63 and 62 are incident onPSD's 58 and 57. Note that the number of PSD's required for OOPmeasurements in the embodiment of FIG. 3 (two) is the same numberrequired by the embodiment of FIG. 1, and one less than the requirednumber in the embodiment of FIG. 2.

Another design alternative for reducing the number of PSD's, gainingmore freedom in their placement, and improving resolution, ismultiplexing the two light sources (emitters). This may be done bymodulating them individually so as to allow them to be electronicallyseparated if their projected beams are simultaneously incident on asingle PSD. The sources can be multiplexed, for example, in time,frequency or wavelength.

Measurement of In-Plane Parameters x, y, and θ

Extension of the techniques described above permits an in-plane sensingsection 70, 68 to be added to any of the four embodiments of the OOPsensing section 10, 11, 12 and 13 (illustrated in FIGS. 1-3 and 5).In-plane sensing section 70, 68 monitors in-plane (planar) parameters ofthe target, a function which is identical in the embodiments of FIGS. 1,2 and 3 and different in FIG. 5 in that target beam 77 is deflectedbefore striking point reflectors 73, 74. This function requires that aplurality of substantially point reflectors 73, 74 be present on thetarget (shown coplanar with the reference plane 72 in the Figures forillustrative purposes only). Point reflectors 73, 74 in preferredembodiments are substantially round and have a surface areasubstantially less than that of the PSD's 82, 83 that receive the beamsreflected from them. In certain preferred embodiments of the presentinvention, a plurality of point reflectors are disposed on asubstantially non-reflective background so as to provide a plurality ofindividual reflected light beams 80, 81 when illuminated by incident(projected) beam 77, 77a from emitter (light source) 75, 75a, beam 77abeing deflected by mirror 78 to become deflected beam 79 beforeilluminating point reflectors.

In in-plane section 70, 68 collimated light beam 77, 77a from singleemitter 75, 75a is incident on special plane reflectors 73 and 74 (afterdeflection by mirror 78 in the case of FIG. 5). Beams 80 and 81reflected from reflectors 73 and 74 are transmitted to PSD's 82, 83.Positions of reflected beams 80 and 81 on PSD's 82, 83 are determinedfrom PSD output signals, and these positions are used in turn tocalculate in-plane displacement and rotation (position) of the sensedobject in space.

Data from light beams 80 and 81 incident on PSD's 82 and 83 can becombined with data from the PSD's associated with one of the OOPsections illustrated in FIGS. 1, 2, 3 or 5 to detect and measure motionof a sensed object in six-DOF. It would be apparent to one of skill inthe art that the single target beam illustrated in the Figures could bereplaced with a plurality of target beams, which may be parallel, andthat are broad enough to illuminate the point reflectors throughout theintended range of motion of the sensed object.

An Embodiment with Combined Functions

Still another embodiment of a six-DOF optical sensing system is shown inFIG. 4. Several elements of the embodiment in FIG. 4 are common to thein-plane parameter measurement sections 70 of FIGS. 1, 2 and 3. There isan additional special plane reflector 71 (another point reflector) and acorresponding PSD 85 on which beam 84, reflected from reflector 71, isincident. Light paths are analogous to those described for in-planesensing section 70 in FIG. 1. Note that in preferred embodiments of thepresent invention using a single emitter 75 (as in FIG. 4), the sensedobject has at least three point reflectors disposed on a substantiallynonreflective background. These point reflectors may be coplanar butneed not be. Further, projected light beams are incident obliquely onthe point reflectors in preferred embodiments. In the case of coplanarpoint reflectors, reflection of a collimated light beam, whetherdeflected or projected directly, results in at least three parallelreflected light beams.

Thus, each embodiment of the present invention illustrated in FIGS. 1-5employs a plurality of reflected light beams incident on one or morePSD's to provide data which can be transformed into the three OOP andthe three in-plane parameters describing motion or position of thesensed object.

Reflecting Surfaces on the Sensed Object

Modification of the sensed or target object to provide the reflectivesurfaces necessary to implement any of the preferred embodiments iseasily accomplished. It is standard procedure in circuit board andsemiconductor fabrication, for example, to place alignment features on apart to permit precise determination of part position. The featuresneeded for this sensing system can be placed very accurately duringfabrication and can be quite small; their placement can be made anintegral part of the manufacturing process. For example, planereflectors 71, 73 and 74 may be applied as a thin film comprising anoptically absorbing material surrounding two small reflective features.While FIGS. 1-3 show separate sections for the OOP and in-planemeasurements, the invention can be made more compact by combining theminto a single unit with a single set of reflective targets (assumingtarget motion is sufficiently small). In this case, time, frequency orwavelength multiplexing could be used to separate the OOP and in-planemeasurements in data processing.

An alternative to a film target would be to integrate the target intothe part itself. For example, one might cover a metal part in two areasand spray a thin coating of light-absorbing material onto the part,producing a part which has a light-absorbing area with two distinct,accurately placed reflectors embedded therein. For a part made from amaterial which is itself a poor reflector, one could deposit two smallmetal reflectors onto the part.

Differences Between In-Plane and OOP Measurements

In-plane parameter measurement is different from OOP parametermeasurement in a fundamental way. For the in-plane measurement sectionit is desirable that a relatively large area of the sensed object beilluminated simultaneously, whereas for the OOP section it is desirablethat the impinging beam diameter be small. Since lateral effectphotodiodes respond to the centroid of a light spot, the size of thereflectors and the beam diameter do not directly establish theresolution of the in-plane sensing section. Hence, it is not onlyfeasible but desirable to use a single light source for the in-planesection of the sensing system, or to use two light sources and a singlelateral effect photodiode for OOP motion sensing. However, the reflectorsize and source beam diameter do determine the range of operation, sincethe entire light spot must be on the sensor. Further, the size of thereflector can indirectly affect the resolution because it can determinethe light intensity hitting the PSD. Greater light intensity produces ahigher signal-to-noise ratio and better sensor resolution until thesource level is so high that the sensors saturate. A single highlycollimated light source (emitter), such as a light-emitting diode (LED),is used for in-plane measurements in preferred embodiments of thepresent invention, while a semiconductor laser or light projected from afiber are the preferred sources for narrow beams required for the OOPsection.

Multiplexing Techniques

Position measurements of multiple light beams irradiating a singletwo-dimensional lateral-effect PSD (see PSD 18 in FIG. 5) can be madesimultaneously through time, frequency or wavelength multiplexing. FIG.6 is a schematic diagram illustrating a system for multiplexing twoposition measurements derived from light beams 114, 116 impinging on PSD118. Light beams 114, 116 originate from emitter 1 and emitter 2, 110,112 respectively, which are in turn modulated by modulator 1 andmodulator 2, 102, 104 respectively, the modulators communicating withthe emitters through links 106, 108 respectively. PSD 118 output is amultiplexed signal carried via link 119 to demodulator 120. Demodulator120 outputs 122, 124 comprise reconstructed (demodulated) signals whichrepresent the respective positions of light beams 114, 116 (from emitter1 and emitter 2, 110, 112 respectively) on PSD 118. Signals 122, 124 arestored in the memory of computer 126 until needed for calculationsrelating positions of light beams 114, 116 on PSD 118 to other variablesrequired for position measurement of a sensed object.

By analogous methods, several light sources may each be modulated atdifferent frequencies and then demodulated in the sensor signalprocessing circuit using, for example, a pulse amplitude modulation(PAM) scheme. The position of each light spot can be determined even ifthere are other beams irradiating the PSD at the same time, and PAM isadvantageous because it is one of the simplest modulation schemes toimplement. Using PAM, each light source is modulated into a highfrequency pulsed wave and then reflected onto the surface of a PSD. ThePSD output photocurrents are then pulsed signals at the same frequency.These signals are amplified and filtered, and the DC mode is restored byusing sample/hold amplifiers. By carefully synchronizing all of thepulse signals and the sampling commands, the position signals of eachlight source can be restored correctly and without distortion.

Experimental results show that the maximum difference between themeasured positions of one light beam in the presence and absence ofanother beam is less than 0.25% of full scale. Further, this differenceis repeatable and almost uniform throughout the entire PSD workspace;therefore it does not affect the resolution and accuracy of themeasurement if the multiple light beams are always present. Test resultsalso show that using multiple light sources simultaneously causes nochange in the linearity and resolution of the sensor. The number oflight beams which can be used simultaneously is limited only by thebandwidth of the PSD and signal loss considerations.

Thus, the main advantage of multiplexing is that the number of PSDsrequired in the presently claimed multidimensional sensing system can bereduced, while the signal processing circuitry is simultaneouslysimplified. The resulting system is more compact, and the changeslargely eliminate alignment difficulties. Further, the effect ofenvironmental variations is minimized as the number of PSD's is reduced.

The PAM technique described above is simply one example of amultiplexing technique that can be used to reduce the number of requiredsensors and sources for the present invention. A second easily describedmodulation approach, equally useful in this application, uses multiplesignals, each pulsed off and on at the same frequency. By delaying thesignals relative to one another, one could arrange to have at most asingle light source active at any given time. This approach falls underthe general heading of time multiplexing. A wide variety of additionalmultiplexing approaches, including frequency, time and wavelengthmultiplexing, are also suitable for this invention.

It is well known that when a light beam is projected onto the surface ofa p-n junction, a photopotential is produced on each plane of thejunction. The photocurrent generated on the surface will flow laterallytoward the electrodes on the boundary C because of the photopotentialgradient in the lateral direction. This phenomenon is called the lateralphotoeffect. The fundamental mechanism of the lateral photoeffect hasbeen discussed in many papers.

The semiconductor PSD based on the lateral photoeffect has been wellstudied and applied extensively in various areas of optical inspectionand measurement since it was first described by J. T. Wallmark ("A NewSemiconductor Photocell Using Lateral Photoeffect," Proc. IRE,45:474-483 (1957)). Several types of PSDs have been developed, such asthe duolateral, pincushion and clover types. Both two-dimensional andone-dimensional versions of the devices are commercially available.

The lateral effect sensor can measure displacement in a spatiallycontinuous manner, unlike other types of large-sensitive-area detectorssuch as charge-coupled devices. Because of the high linearity, goodresolution, and fast response, the potential applications of PSDs arequite broad. For the present invention, all two-dimensional PSD's aresuitable, although some have higher resolution or linearity and othershave lower cost.

Normally, one PSD is needed to measure the displacement of each lightbeam. However, in an optical measurement system which uses multiplebeams and multiple detectors, such as the precision multipledegree-of-freedom motion measurement system suggested by W. Wang("Design Of An Optically Based Sensing System For Magnetically LevitatedMicro-Automation Systems," Ph.D. Dissertation, Mechanical EngineeringDepartment, University of Texas at Austin, December, 1989), the hardwarecan become large and redundant. Because of the combined cost of many PSDsensing elements, the system cost may become prohibitively high.Further, alignment difficulties tend to increase dramatically as sensingelements are added.

These problems can sometimes be avoided using a PSD array, but the arraysensor has a low resolution (depending on the width of each pixel andthe gap between pixels), and it is usually more difficult to fabricate.Therefore it is highly desirable to use a single PSD to measure thedisplacements of several light beams instead of using one PSD for eachlight beam. The resulting advantages are obvious: a more compact system,lower cost, faster calibration, and preservation of almost identicalphysical and environmental conditions for each measurand.

Borrowing from technology widely used in the telecommunication industry,a modulation method can be implemented to permit one sensor to monitormultiple light sources. For example, each light source may either bemodulated at a different frequency or delayed in time so that only onesource is on at a given instant. The position signals from the PSDsensor at each instant are superpositions of the position signalsgenerated by each active source. The information on a specific lightsource location can be obtained through demodulation. Other modulationapproaches besides the examples cited here may also be usedsuccessfully.

Kinematic Transformations

Transformations have been derived to calculate position and motion of atarget object based on information supplied by the structuresillustrated in FIGS. 1 and 3. Transformations for alternativeembodiments of the present invention can be derived by one of skill inthe art. Typical equations for the out-of-plane sections 11, 12 of FIGS.1 and 3, respectively, are set forth below: ##EQU1## S.sub.α is thedistance between the pivot point of reflective surface 69 and theintersection of light beam 23 in FIG. 1 or light beam 60 in FIG. 3 withreflective surface 69, when reflective surface 69 and PSD (29, 30, 57,58) are parallel. Z is measured at the pivot point of reflective surface69. The pivot point is the point about which the reflective surfacerotates in the φ and ψ directions, and it is the intersection of the xand y axes of the system.

The following equations apply to the embodiment illustrated in FIG. 1:

    a.sub.11 =y.sub.1 tan.sup.2 α

    a.sub.12 =[y.sub.1 tanα+d (tan.sup.2 α+1)] sin (x.sub.1)

    a.sub.21 =2 tanα

    a.sub.22 =[tan.sup.2 α-1] sin (x.sub.1)

    b.sub.11 =y.sub.2 tan.sup.2 β

    b.sub.12 =[y.sub.2 tanβ+d (tan.sup.2 β+1)] sin (x.sub.2)

    b.sub.21 =2 tanβ

    b.sub.22 =[tan.sup.2 β-1] sin (x.sub.2)

    c.sub.11 =x.sub.1

    c.sub.12 =d

    c.sub.21 =0

    c.sub.22 =1

    d.sub.11 =x.sub.2

    d.sub.12 =d

    d.sub.21 =0

    d.sub.22 =1

where:

α is the angle between light beam 23 and the plane of PSD 29;

β is the angle between light beam 24 and the plane of PSD 30;

x₁ and y₁ are measurements obtained from PSD 29;

x₂ and y₂ are measurements obtained from PSD 30; and

d is the initial distance between the reference plane and the sensedplane.

The following equations apply to the embodiment illustrated in FIG. 3:

    a.sub.11 =y.sub.1 tan.sup.2 α

    a.sub.12 =[y.sub.1 tanα+d (tan.sup.2 α+1)] sin (x.sub.1)

    a.sub.21 =2 tanα

    a.sub.22 =[tan.sup.2 α-1] sin (x.sub.1)

    b.sub.11 =y.sub.2

    b.sub.12 =d

    b.sub.21 =0

    b.sub.22 =1

    c.sub.11 =x.sub.1

    c.sub.12 =d

    c.sub.21 =0

    c.sub.22 =1

    d.sub.11 =x.sub.2 tan.sup.2 β

    d.sub.12 =[x.sub.2 tanβ+d (tan.sup.2 β+1)] sin (y.sub.2)

    d.sub.21 =2 tanβ

    d.sub.22 =[tan.sup.2 β-1] sin (y.sub.2)

where:

α is the angle between the first light beam 60 and the plane of PSD 58;

β is the angle between the second light beam 61 and the plane of PSD 57;

x₁ and y₁ are measurements obtained from the first PSD 58;

x₂ and y₂ are measurements obtained from the second PSD 57; and

d is the initial distance between the reference plane and the sensedplane.

What is claimed is:
 1. An improved method for measuring position of asensed object in space without physically contacting the object,comprising the steps of:providing on a sensed object a first reflectivesurface and an in-plane target, the in-plane target comprising aplurality of point reflectors on a substantially non-reflectivebackground; emitting a first projected beam of light from a firstemitter to impinge upon the first reflective surface; emitting a secondprojected beam of light from a second emitter to impinge upon the firstreflective surface; reflecting the first projected beam of light fromthe first reflective surface to form a first reflected beam; reflectingthe second projected beam of light from the first reflective surface toform a second reflected beam; determining a position of the firstreflected beam and a position of the second reflected beam; projecting atarget beam of light adapted to impinge upon the in-plane target;reflecting the target beam of light from the point reflectors to producea plurality of target-reflected beams; determining positions of thetarget-reflected beams; and calculating the position of the sensedobject in space using the determined positions of the first and secondreflected beams and target-reflected beams.
 2. The method of claim 1,wherein the first projected beam and second projected beam aresubstantially parallel.
 3. The method of claim 2, wherein the firstprojected beam and second projected beam are oblique to the firstreflective surface.
 4. The method of claim 1, wherein the firstprojected beam is deflected by a mirror prior to impinging on the firstreflective surface.
 5. The method of claim 4, whereinthe first projectedbeam is deflected and split into a plurality of split first beams thatimpinge upon the first reflective surface at different locations;reflecting the first projected beam comprises reflecting the split firstbeams from the first reflective surface to form a plurality of splitfirst reflected beams; and determining a position of the first reflectedbeam comprises determining positions of the split first reflected beams.6. The method of claim 5, wherein the split first beams aresubstantially parallel.
 7. The method of claim 1, wherein the firstprojected beam and second projected beam are substantially nonparallel.8. The method of claim 1, wherein the first reflective surface iscontinuous and planar.
 9. The method of claim 1, wherein projecting atarget beam of light comprises projecting a collimated target beam oflight.
 10. The method of claim 1 wherein the positions of the firstreflected beam, second reflected beam, and target-reflected beams aredetermined using position-sensitive-detectors.
 11. The method of claim 5wherein the positions of the split first reflected beams, the secondreflected beams, and the target-reflected beams are determined usingposition-sensitive-detectors.
 12. The method of claim 10, furthercomprising the steps of:multiplexing the first projected beam; andmultiplexing the second projected beam;such that the first and secondreflected beams can be distinguished when impinging on the positionsensitive detectors.
 13. The method of claim 12, wherein the positionsof the first and second reflected beams are determined using a singleposition-sensitive-detector, and further comprising the step ofdistinguishing the position of the first reflected beam from theposition of the second reflected beam.
 14. A system for sensing positionof a sensed object in space, the sensed object having a first reflectivesurface and an in-plane target located on the sensed object, thein-plane target comprising a plurality of point reflectors on asubstantially non-reflective background, the system comprising:first andsecond beam emitters for projecting beams of light to impinge upon thefirst reflective surface, thereby forming a first reflected beam and asecond reflected beam; a first reflected beam detector positioned tointercept the first reflected beam for determining a position of thefirst reflected beam; a second reflected beam detector positioned tointercept the second reflected beam for determining a position of thesecond reflected beam; a target beam emitter for projecting a targetbeam to impinge upon an in-plane target, whereby a plurality ofreflected-target beams are produced by reflection from the pointreflectors; and reflected-target beam detectors positioned to interceptthe reflected-target beams for determining their positions.
 15. Thesystem of claim 14, further comprising a digital computer coupled to thefirst reflected beam detector, the second reflected beam detector andthe reflected-target beam detectors, the computer being adapted tocalculate the position of the target object in space using the positionsof the first and second reflected beams and the positions of thereflected-target beams.
 16. A system for sensing position of a sensedobject in space, the sensed object having a first reflective surface andan in-plane target located on the sensed object, the target comprising aplurality of point reflectors on a substantially non-reflectivebackground, the system comprising:a first beam emitter for projecting abeam of light to impinge upon a plurality of mirrors, said mirrorssplitting and deflecting the first beam into a plurality of split firstbeams which impinge on the first reflective surface, thereby creatingsplit first reflected beams; a second beam emitter for projecting a beamof light to impinge upon the first reflective surface, thereby creatinga second reflected beam; split first reflected beam detectors positionedto intercept the split first reflected beams for determining a positionof the split first reflected beams; a second reflected beam detectorpositioned to intercept the second reflected beam for determining aposition of the second reflected beam; a target beam emitter forprojecting a target beam to impinge upon the in-plane target to producea plurality of reflected-target beams by reflection from the pointreflectors; and reflected-target beam detectors positioned to intercepteach reflected-target beam for determining a reflected-target beamposition.
 17. The system of claim 16, further comprising a digitalcomputer coupled to the split first reflected beam detectors, the secondreflected beam detector, and the reflected-target beam detectors, thecomputer being adapted to calculate the position of the target object inspace using the position of each beam.